Preparation,Properties And Mechanism Investigation Of Viscoelastic Surfactant Self-Diverting Acid

A large number of low permeability carbonate oil and gas fields are distributed in China,and it is difficult to exploit them.The acid fracturing technique is an important measure to develop low permeability carbonate reservoirs.When acid fracturing operation is carried out with conventional acid,the acid will first enter the high permeability layer,leaving the low permeability layer unstimulated.When viscoelastic surfactant self-diverting acid is injected into formation,its viscosity will increase rapidly with the acid-rock reaction,which can reduce the filtration of acid,block the acid wormhole,make subsequent acid enter into the low permeability zone and achieve the goal of uniform aciding.Therefore,the research on the theory and application of viscoelastic surfactant self-diverting acid has become a hotspot.In this paper,amide intermediates were prepared through amidation using erucic acid,oleic acid,soybean oil and N,N-dimethyl-1,3-propanediamine as raw materials.Betaines were prepared through quaterisation using amide intermediates,sodium chloroacetate and 3-Chloro-2-hydroxypropanesulfonic acid sodium.A nonionic viscoelastic surfactant is also synthesized.The structures of the product were characterized by NMR and FTIR.The viscosity of self-diverting acid at different stages of acid rock reaction was tested.The experimental results showed that the viscosity of acid system increased with the acid rock reaction.The maximum viscosities of all the spent acids were over 300 mPa·s.The viscosity changes of EACB self-diverting acid and NVES self-diverting acid occured in the main process of acid rock reaction.And the viscosity changes of other self-diverting acids occured at the spent acid stages.The rheological properties of self-diverting acid were studied systematically,including steady-state shear test,small oscillation test and thixotropy test.The experimental results indicated that the fresh acid of EACB self-diverting acid showed the shear-thinning behavior and the fresh acids of other self-diverting acids presented the characteristics of Newtonian fluids,which indicated that the micelles in the fresh acid of EACB self-diverting acid are rodlike micelles and the micelles in the fresh acid of other self-diverting acids are spherical micelles.The plateau moduli and the relaxation times of all viscoelastic surfactant self-diverting acids increased gradually with the acid rock reaction,which showed that the viscoelastic properties of the systems were enhanced gradually.When the hydrochloric acid in the systems was completely consumed,the viscoelasticity of the systems reached the strongest and wormlike micelles were present in the systems.Thixotropy research showed that the viscosity of all spent acids of self-diverting acid systems cannot be fully recovered after high shear.And the stronger the viscoelasticity,the longer the recovery time.Once again,there were wormlike micelles in the spent acid system.Through the study on the structure and size of micelles in the self-diverting acid systems,the size of the micelles in all the self-diverting acid systems increased with the acid rock reaction.And in all the spent acid systems,there were wormlike micelles.Therefore,the internal reason of the change of the rheological property of the acid systems was the change of the structure and size of micelles in the systems.